How to layer movie clips with black backgrounds in compositor?
Two ways spring immediately to mind, assuming zero emission backgrounds:


Take the output of the image and roll it through a node to get the maximum value.
Set Alpha on the image from the result from 1.1.
Use a generic alpha over and done.


For a more abstract glowy vibe:


Take the output of the image and roll it through a colour to BW node.
Set Alpha on the image from result from 1.1
Use a generic alpha over and done.


The former is a generic alpha over on imagery with zero emission background. The latter is a generic alpha over, but will result in some values being semi-additive akin to fire / candle flames.